My invention is adapted particularly for use in a rotary valve assembly of a power steering mechanism of the kind disclosed in my U.S. Pat. No. 4,570,736, which is assigned to the assignee of my invention. The mechanism disclosed in the '736 patent includes a rotary valve assembly comprising an inner rotary valve member and an outer valve sleeve. The valve sleeve is surrounded by a valve housing. A high pressure port in the valve housing communicates with a high pressure valve groove in the valve sleeve. Internal valve lands in the valve sleeve register with external valve lands on the inner valve member to control pressure distribution to each of two pressure ports in the valve sleeve, which communicate with opposite sides of a pressure operated piston of a fluid motor that provides the power assist for a steering gear. One port is pressurized relative to the other to effect a right turn. The pressure differential is reversed to effect a left turn. The right turn port and the left turn port are sealed from the high pressure supply port by a seal ring, which sealingly engages the valve housing. The seal rings are located on each side of the pressure port in the valve sleeve.
The steering mechanism of the '736 patent includes also a high speed pressure port that is pressurized when the vehicle speed exceeds a calibrated value. At low speeds, that pressure port is de-pressurized. The seal between the high speed port and the adjacent twin port will be subjected to a differential pressure in one axial direction at low speeds and in the opposite direction at high speeds. There is a tendency, therefore for the seal ring to shift under the influence of an increasing pressure in either the left turn passage or the right turn passage. The seal ring thus is incapable of preventing leakage flow across the seal ring through the clearance spaces between the seal ring and the seal ring groove in which it is situated. It is recent design practice to try to minimize leakage flow by providing a close tolerance fit between the seal rings and the grooves. This effectiveness in reducing leakage in this manner, however, is not satisfactory. Also, the close tolerances that are required increase the manufacturing cost.
The sealing action of such known designs is even more imprecise if the coefficient of thermal expansion of the seal material and the groove material are not approximately equal.
This leakage flow reduces the responsiveness of the steering gear mechanism as steering torque is applied to the inner valve member. It also makes it necessary to provide flow compensation to effect turning maneuvers of the vehicle, thus requiring the use of a power steering pump of increased capacity which, in turn, increases the total parasitic power loss for the vehicle engine.
I am aware of prior art teachings that deal with means for preloading seal rings on reciprocating members such as a piston or reciprocating valve elements. These seal rings usually comprise an inner spring member that acts radially outwardly on a seal ring to establish sealing contact between the reciprocating member and the surrounding cylinder. Examples of piston packings having internal springs for establishing a sealing force on the piston rings may be seen by referring to U.S. Pat. Nos. 1,640,155 and 1,735,596.
I am aware also of reciprocating piston seals for use in power steering gears, but these are used to establish a seal for a piston of the fluid motor used to obtain power assist. An example of this application of a piston ring seal with an internal spring for effecting sealing engagement of the seal with respect to the cylinder is shown in U.S. Pat. No. 2,962,331, where an inner spring located in a seal ring groove acts on a surrounding O-ring to urge the O-ring into sealing engagement with the surrounding cylinder member of the fluid motor for the steering gear.
U.S. Pat. No. 4,793,433 shows an application similar to that of the seal disclosed in the '331 patent. It comprises a Teflon ring and an O-ring fitted in an annular groove in such a way that the Teflon ring exerts a radial force on the O-ring to establish sealing engagement with the surrounding cylinder wall.
None of these seal assemblies of the prior art mechanisms is capable of establishing fluid pressure isolation of a pressure port, such as the high speed pressure disclosed in the '736 patent, and adjacent pressure delivery ports in a rotary valve mechanism wherein the pressure balance across the seal ring is subject to change upon rotary adjustment of the inner member of the rotary valve with respect to the valve sleeve.